An organic light-emitting diode device, also called an OLED, commonly includes an anode, a cathode, and an organic electroluminescent (EL) unit sandwiched between the anode and the cathode. The organic EL unit includes at least a hole-transporting layer (HTL), a light-emitting layer (LEL), and an electron-transporting layer (ETL). OLEDs are attractive because of their low drive voltage, high luminance, wide viewing-angle and capability for full-color displays and for other applications. Tang et al. described this multilayer OLED in their U.S. Pat. Nos. 4,769,292 and 4,885,211.
OLEDs can emit different colors, such as red, green, blue, or white, depending on the emitting property of its LEL. There is an increasing demand for broadband OLEDs to be incorporated into various applications, such as a solid-state lighting source or a full-color display. By broadband emission, it is meant that an OLED emits sufficiently wide light throughout the visible spectrum so that such light can be used in conjunction with filters to produce a full-color display. In particular, there is a need for white light OLEDs (or white OLEDs) where there is substantial emission in the red, green, and blue portions of the spectrum, wherein a white-emitting EL layer can be used to form a multicolor device in conjunction with filters. The emission of a white OLED device generally has 1931 Commission Internationale d'Eclairage (CIE) chromaticity coordinates, (CIEx, CIEy), of about (0.33, 0.33). White OLEDs have been reported in the prior art, such as reported by Kido et al. in Applied Physics Letters, 64, 815 (1994), J. Shi et al. in U.S. Pat. No. 5,683,823, Sato et al. in JP 07-142169, Deshpande et al. in Applied Physics Letters, 75, 888 (1999), and Tokito et al. in Applied Physics Letters, 83, 2459 (2003).
In order to achieve broadband emission from an OLED, more than one type of molecule has to be excited because each type of molecule only emits light with a relatively narrow spectrum under normal conditions. A LEL comprising a host material and one or more than one luminescent dopant can achieve light emission from both the host and the dopant(s) resulting in a broadband emission in the visible spectrum if the energy transfer from the host material to the dopant(s) is incomplete. However, a white OLED with only one LEL will have neither a wide enough emission covering the whole visible spectrum, nor will it have a high luminance efficiency. A white OLED having two LELs can have better color as well as better luminance efficiency than the device having one LEL. However, it is difficult to achieve a wide emission with a balanced intensity from red, green, and blue colors because a white OLED having two LELs typically has only two intensive emission peaks. For example, in a commonly used white OLED having two LELs, if the colors of the LELs are yellow and greenish blue, the red primary color emission will be weak in the device; if the colors of the two LELs are red and greenish blue, the green primary color emission will be weak in the device; and if the colors of the LELs are green and red, the blue primary color will be weak. A white OLED having three LELs of different color was also proposed but it is still difficult to achieve a wide emission from the device because the most intensive light typically comes from the LEL with a dopant having the narrowest optical band gap and the emission spectrum shifts with different drive conditions.
In a full-color display using white OLEDs as the pixels, the perceived red, green, or blue color from the human eyes comes from the pixels with a red, green, or blue color filter on top of the pixels, respectively. If each of the white OLED pixels in the display has an emission including balanced red, green, and blue primary color components, the light intensity passing through the color filter is about one third of the white emission intensity. However, if the white OLED pixels do not have balanced red, green, and blue emission, one of the primary color components will have the intensity lower than one third of the white emission intensity after passing through the color filter. As a result, in order to achieve a comparable emission intensity of the specific primary color, the corresponding white OLED pixel has to be driven under higher current density causing higher power consumption and a shorter lifetime. Therefore, color compensation is needed for a conventional white OLED to achieve balanced red, green, and blue emission.
In order to improve the full color emission of an OLED, stacked OLEDs have been fabricated as disclosed by Forrest et al. in U.S. Pat. No. 5,703,436. These stacked OLEDs are fabricated by vertically stacking multiple, individually addressable OLED units, each emitting light of a different color, and wherein intra-electrodes are provided between each of the vertically stacked OLED units as a way of independently controlling the emission from each individual OLED unit in the OLED device. As a result, full color emission as well as a balanced white color emission is readily achieved. While this permits for improved color emission and a larger emission area compared to conventional full-color OLEDs, the overall construction of the OLED is complex, requiring transparent electrodes, additional bus lines for providing electrical power, as well as a separate power source for each of the stacked OLED units.
Another type of stacked OLED (or tandem OLED, or cascaded OLED) structure used for EL improvement, which is fabricated by stacking several individual OLEDs vertically and driven by only a single power source, has been proposed or fabricated by Jones et al. in U.S. Pat. No. 6,337,492, Tanaka et al. in U.S. Pat. No. 6,107,734, Kido et al. in JP Patent Publication 2003045676A and in U.S. Patent Publication 2003/0189401 A1, and Liao et al. in U.S. Pat. No. 6,717,358 and U.S. Patent Application Publication 2003/0170491 A1, the disclosures of which are herein incorporated by reference. Matsumoto et al., reported in SID 03 Digest, 979 (2003) that a tandem white OLED can be constructed by connecting a greenish blue EL unit and an orange EL unit in the device, and white light emission can be achieved by driving this device with a single power source. While luminance efficiency can be increased, this tandem white OLED device has weaker green and red color components in the spectrum. In U.S. Patent Application Publication 2003/0170491 A1, Liao and Tang proposed a tandem white OLED structure by connecting a red EL unit, a green EL unit, and a blue EL unit in series within the device. When the tandem white OLED is driven by a single power source, white light emission can be formed by spectral combination from the red, green, and blue EL units. While color emission and luminance efficiency can be improved, this tandem white OLED cannot be made with less than three EL units, requiring that the drive voltage be at least 3 times as high as that of a conventional OLED.